Julie DeNeen was raised by her biological mother and a step-father who adopted her after DeNeen's birth father relinquished his legal rights. But she yearned for the father she never knew, wondering why he abandoned her.

"I had no picture and no contact with my biological father," said DeNeen, now 31 and married with three children in Clinton, Conn. "I hardly knew he existed."

At 13, DeNeen sought out and found her father, but after a few visits they grew apart. "I viewed him as a strange relative," she said "I wasn't prepared … and it was just awkward."

But in 2011, again obsessing over her family roots and wanting her children -- 8, 6 and 5 – to know their grandfather, DeNeen wrote him a letter, telling him she was sorry "I had fallen off the planet" and that she loved him and wanted him to "be a father figure in my life."

So they reconnected, but what evolved was far from a healthy father-daughter relationship – amidst her grief and longing, sexual sparks flew and it nearly destroyed her 10-year marriage and a fragile new bond with her biological father.

"I realized how similar we were … We could finish each other's sentences," she said. "It was a combination of elations. And there was the adrenaline and on top of the grief, thinking why can't you go back in time. And in that combination of grief and need and feeling that you fit with someone, you get a concoction that made things very confusing."

"I had this strange falling in love feeling, holding my Dad's hand," said DeNeen. "It wasn't like a daughter, it was like something else."

Psychologists say that taboo is normally in place when family members grow up in close proximity by virtue of reverse sexual imprinting, or the Westermarck effect, which desensitizes them to later sexual attraction. Researchers hypothesize it evolved so biological relatives would not inbreed.

The phenomenon was first identified by Barbara Gonyo in the 1980s. She wrote a book, "I'm His Mother, But He's Not My Son," that recounted her personal story of reuniting and having sexual feelings for a son whom she had placed for adoption when she was 16. Gonyo fell in love -- a byproduct of delayed bonding that would normally have taken place in infancy, had they not been separated by adoption.

Gonyo, now a retired grandmother, created an online support group and DeNeen, who has a background in psychology, has taken up her work on a new website that she launched just two weeks ago, educating and intervening when others fall into the dangerous emotional trap of GSA.

When family members who are strangers finally meet as adults, the brain struggles to associate each other as family and, instead, they become "captivated" with one another, according to DeNeen.

"Parent and child go through a very complex bonding process from the beginning of life to the first six years," she said. "They go through phases and in the teen years, they separate. That whole process goes dormant until they reunite as adults. It's almost like it awakens back the recognition in that the other person is a mirror of yourself."

DeNeen said she felt like she was regressing back to childhood, falling in love and looking to her dad as a hero. "I felt a lot of need for intimacy," she said. "The lines were so blurry."

But she makes it clear that she never had sexual intercourse with her father, even though the relationship was "very inappropriate." And like others who experience GSA, she crossed physical boundaries that were "embarrassing, confusing, amazing and overwhelming,"

Many Therapists Are Unfamiliar With GSA

And because of that, mental health experts are not experienced in helping patients. They often mistakenly confuse GSA with incest or sexual abuse, shaming adoptees.

"They just don't have the training and the topic is completely foreign," she said.

Alvarado, who has treated four families with GSA, also blames the adoption process itself.

"It fuels the secrecy and builds up the fantasy about what the other family might be like," she said. "It is mitigated when you have open access to records and birth certificates and the family from infancy is included."

The parent is often as smitten as the child, experiencing these feelings for the first time, and is rarely a sexual predator, according to Alvarado. But the imbalance of power makes the adult child more vulnerable to manipulation.

In DeNeen's case, her feelings "stoked the fire" in her father. "I wasn't aware at the beginning. When I said, 'I love you so much, Dad,' it didn't dawn on me the kind of confusion it would create in his mind. He was feeling the need to bond, too and the only way as an adult is through sexual intimacy."

Those with GSA can be healed, according to Alvarado, and the relationship can continue with nonjudgmental therapy and "normalization" of their feelings -- just in a different form.

DeNeen credits Carly Sullens, another adoptee who was writing a blog on the genetic sexual attraction website about her own birth father, for giving her the courage to seek help.

Together, they have now created an interactive online forum for others, hoping to give them the tools and to intervene before it is too late. So far, the blog has more than 50,000 views and 40 members struggling with GSA have subscribed to the forum.

Sullens, a 39-year-old art therapist and childbirth educator from Florida, embarked on a similarly forbidden relationship with her biological father about the same time as DeNeen. After discovering each other online, they became close friends.

"GSA doesn't just affect the two people, but the whole family system," said Sullens, who was in a 10-year loving relationship with her husband and had two children, now 6 and 9.

"It's not that I had a bad upbringing or wasn't able to attach to my adopted parents – I did, very much so," she said. "That's why it's so confusing when it happens."

Sullens was adopted at the age of 4 months. Her father died several years ago and just last year, her mother was diagnosed with a painful cancer. As she made visits to her dying mother, Sullens was drawn to seek out her birth father, who lived in the same town.

"Something just snapped and broke in me," she said. "I felt the urge to attach to him, much like a young child would. I thought it would make everything better."

"He did everything a good parent would and said, 'I will be there for you and if there is anything you need, let me know," said Sullens. "He was very convincing and supportive. I think I overly attached, wanting to keep my grief at bay."

Sullens said that like DeNeen, she never had sex with her father, but their interactions were also "inappropriate."

"I wanted to sit with him like a little girl with her dad," she said. "I remember falling into his arms. And when I was close to him my body would physically respond. It was not always arousal, but as someone who was adopted, I never was near my genealogy and it was almost as if there was a relaxation response."

Sullens said her actions were so out of character. "I have never looked at a man outside marriage and never wanted to," she said. "But all the old wounds flew open and I lost my footing."

The relationship lasted only a couple months, when she realized "it wasn't right," Sullens began scouring the Internet for answers and began to blog on Gonyo's online forum. DeNeen, going through the same experience, found Sullens and they realized they weren't alone.

For Sullens, the experience ended in trauma, losing her father and all of her biological relatives for a second time. Every time she broached the topic of GSA and urged her father to seek help, he would "close the door and stop talking to me for weeks."

"Eventually, he cleaned the slate and blamed everything on me," she said. 'He didn't take any part of the responsibility and then he threw me under the bus to everyone … In my heart, the most painful part of my story is the rejection. I look back and even the other parts look sick, but don't hurt me as much as the rejection."

Today, she has no relationship with her father or her half-siblings and a full sister. But, with DeNeen, she reaches out to help others. In their first public appearance, they appeared in May on the Dr. Drew show.

"GSA is such a vortex and you lose your sense of reality – your true north is gone," said Sullens. "If someone doesn't wish to reground, they get lost quickly. Julie and I did it for each other … "Now we want to provide this to others."

With hard work in therapy and support from each other, both their marriages have survived the tumult.

DeNeen was honest with her husband from the start and said when she began to feel "cut in half," she persuaded her father to go into intense therapy. She did the same.

"Part of our message is that it is very normal to have those feelings, but when you take those feelings and act on them, it can be incredibly destructive and very traumatic for both parties," she said. "It's a very difficult road."

Surprisingly, DeNeen has been able to keep the healthy side of her relationship with her father alive -- a feat that is rare among those who have experienced GSA.

They only talk by telephone today, but plan to see each other again – always with others present.

"We have tried to work through all the issues that propelled us into this toxic situation," said DeNeen. "I think it's very possible we can have a normal father-daughter relationship if we are willing to work at it. It's rare, but possible."